Polyethylene resins having bimodal characteristics include resins that comprise two components having different properties, such as for instance two components of different molecular weight, i.e. a component with a relatively higher molecular weight component (HMW) and a component with a lower molecular weight (LMW) component; two components of different densities; and/or two components having different productivities or reaction rates with respect to co-monomer.
The use of metallocene catalysts for polymerization and copolymerization of ethylene is a relatively recent development. Processes for producing bimodal polyolefins in general and bimodal polyethylene in particular in the presence of metallocene catalysts have been described.
Bimodal polyethylene resins can be prepared according to different methods. Bimodal polyethylene products can for instance be made by physically blending different monomodal polyethylene products, which are independently produced. However, a problem with those physically produced bimodal products is that they usually contain high levels of gels.
Bimodal polyethylene can also be produced by combining two different catalyst systems in a single reactor, as is described for instance in WO 2006/045738. Alternatively, a single dual site catalyst system can be used to produce bimodal polyethylene in a single reactor, as is described for instance in WO 2004/029101.
In addition to producing bimodal polyethylene in a single reactor, bimodal polyethylene can also be produced in serially connected reactors. For instance, WO 02/28922 describes a process for the production of bimodal polyethylene comprising producing a first polyethylene fraction in a first slurry loop reactor and producing a second polyethylene fraction in a second slurry loop reactor, serially connected to the first reactor, wherein the first polyethylene fraction is passed from the first reactor to the second reactor, and wherein the molecular weight of the polyethylene fractions in the first and second reactors is different. Depending on the desired product characteristics, catalysts features and/or reaction conditions can be adapted in such processes.
However, using metallocene-based catalyst systems to catalyze the preparation of bimodal polyolefins such as bimodal polyethylene, results in polymer fractions that may be difficult to mix with one another, in particular when bimodal polyolefins are produced in separate reactors. A problem associated with known bimodal polyethylene products is that if the individual polyethylene components are too different in molecular weight and density, they may not be homogeneously mixed with each other as desired. As a consequence harsh extrusion conditions or repeated extrusions are sometimes necessary which might lead to partial degradation of the final product and/or additional cost. Thus the optimum mechanical and processing properties are not achieved in the final polyethylene product. Also, bimodal polymer particles produced may not be sufficiently uniform in size, and hence segregation of polymer during storage and transfer can produce non-homogeneous products.
Another problem with known bimodal polyolefins is that defects sometimes are visible in the finished product where the catalyst used in the olefin polymerization is a metallocene catalyst. In particular, dots or specks and/or rough patches sometimes are visible on the surface of products, e.g. tubes or pipe, made from pellets of a bimodal product that were produced using a metallocene catalyst. Such defects can make the pipe weaker and can affect the free flow of liquid through the pipe. This type of defect may occur because of homogenization problems in the extruder.
Thus, many applications still require improved miscibility of the polyolefin components of a bimodal polyolefin product such that in turn the mechanical and processing properties of the polyolefins, and in particular polyethylene, can be further improved.
In view of the above, there remains a need in the art to provide an improved polymerization process for making bimodal polyolefin resin, and in particular polyethylene, which overcomes at least some of the above-mentioned problems.